Statement
London’s Exotic Flora

“The wild flowers of our towns and cities would be a drab lot if it weren’t for these immigrants. For they’re not only fascinating echoes of the human life in urban areas, but a vastly colourful collection.” (Richard Mabey, 1973)

“Our flora has gone through a revolution as great as the colonization of these islands after the Ice Age. The change has been very sudden; the majority of these naturalized species have been introduced in the past 200 or 300 years. This is tremendously exciting.” (Martin Ingrouille, 1995)

Introduction
Origins
London plane, London rocket and Londonpride are London plants that are all, perhaps surprisingly, introductions from overseas. Together with a remarkable array of other introduced plants, these floral Londoners form a significant element of London’s vegetation. Twenty of the best known are listed at the end.
More than 40% of plants occurring in the wild that are described in the latest flora of the British Isles (Stace, 1997) have originated from overseas. In London that proportion is considerably exceeded and such introductions are more varied and more significant here than anywhere else in Britain. Coming from many different parts of the world, they provide a truly international dimension to the Capital’s flora and paint a vivid picture of the mixed cultural heritage of our city.
London’s exotic flora has arrived here by many varied routes. A significant number have ‘escaped’ from gardens to become naturalized. Seeds of other plants were inadvertently brought in with imported materials such as birdseed, foodstuffs, timber, minerals and wool waste. As the country’s largest trading and commercial centre, London has been historically rich in such seed sources but today, modern developments such as seed cleaning and containerization, as well as changes in world trade, have reduced this supply.
The ‘heat island effect’
Many introductions come from warmer parts of the world and flourish in London as a consequence of the ‘heat island effect’. In common with other large cities, the central, built-up part of London experiences higher temperatures than the periphery and surrounding countryside as the result of the urban environment’s reflective surfaces and abundance of domestic and industrial heat sources. As a result, the centre of London has a reduced number of frosty nights, longer growing season and higher maximum temperatures in the summer. This brings the urban climate closer to that of the Mediterranean, allowing plants such as London rocket, Chinese mugwort, Guernsey fleabane, hoary mustard, and many other warmth-demanding species to thrive. Many such exotic plants are currently scarce outside London, but with global warming predicted to continue, they may spread to more of London in the future – some are already doing so of their own accord.
Gardens, hybrids and the botanical melting pot
The British are often characterized as a ‘nation of gardeners’ and nowhere is this more apparent than in London, where the number and variety of gardens is a striking feature. Many naturalized exotic London plants derive from such gardens, either as a result of seed dispersal (butterfly bush, michaelmas daisy, cotoneaster, antirrhinum etc.) or as ‘cast-outs’ of more vegetatively vigorous species (Japanese knotweed, ground-elder, Canadian golden-rod, snowberry etc.).
Occasionally such garden-derived species form spontaneous hybrids with their native relatives, and London supports a number of these unusual plants, some of which can be fully fertile. One example is the Highclere holly, now a feature of many London woods, which has arisen through hybridization between our native holly and a species from the Canary Islands. The most widespread bluebell in London, that provides us with such a welcome spring display, is a hybrid derived from the native species and the Spanish bluebell.
Concerns have been raised that this interbreeding might result in the eventual decline of the native parent. Intervention, however, may not always be appropriate or practical in urban London where the woodland flora is simply adapting to its surroundings, and the native and hybrid plants are difficult to tell apart. However, should this process spread to our internationally important native bluebell woods in the wider countryside, for example, prompt action would be needed.
There are other examples of hybridization, such as the sterile hybrid of the probably  native sticky groundsel and the introduced Oxford ragwort, originally named after London (Senecio x londinensis). These events show evolution in action and may sometimes lead to the appearance of new species in our Capital’s botanical ‘melting pot’.

Conservation Considerations
The biodiversity value of exotic flora
It is argued that because native plants in general support a greater variety of animal life than introductions, conservation management should favour the former. This is undoubtedly true as a generality in semi-natural habitats. However, it should also be remembered that there are many introduced plants that provide nectar for insects at times when native sources are in short supply, fruit for birds and mammals and, in the case of many coniferous trees, valuable nesting sites for birds. It is also the case that introductions can sometimes support a considerable biomass of the relatively few invertebrate species that have adapted to them. Sycamore is one well-known example, but there are many others as gardeners often discover to their dismay! Such large invertebrate populations can be an important source of food for insectivorous birds.
Encroaching ‘problem’ species
A small minority of introduced plants continue to cause significant problems in London through their vigorous growth and tendency to encroach on other vegetation. Such species will often need to be controlled where they threaten rare or declining species or their habitats.
Ponds and canals are particularly susceptible to encroaching flora. A notorious trio of aquatic introductions have had a particularly serious impact in recent years: New Zealand pigmyweed, parrot’s-feather and, most recently, floating pennywort. All three are grown by aquarists and get discarded into ponds and other watercourses where they can spread at a remarkable rate. The tropical American water fern can also cause problems, but is rarely as persistent as the above three.
The control of these plants will be addressed as a priority in the relevant Habitat Action Plans. Plantlife is calling on the Government to introduce legislation to prohibit their sale and these proposals should be supported (Harper 2000).
A few terrestrial introductions also need to be controlled in some circumstances. Japanese knotweed and giant hogweed, whose deliberate introduction is outlawed by the Wildlife and Countryside Act (Schedule 9), are particularly well known examples. However, such plants make up only a small minority of London’s introduced flora and it should be remembered that invasiveness is not a characteristic limited to exotics. In certain circumstances bracken, purple moor-grass, holly and sea buckthorn can be encroaching.
We need to balance the undoubted problems that some exotic species can create against the rich diversity, historical and cultural interest and the considerable local distinctiveness that the vast majority bring to the Capital. Most introductions are benign, and in urban areas the natural colonization of brownfield land by native and exotic species has formed communities that are distinctive and unique to cities. London would certainly be a drabber place without them.

Future Action
There are three major actions that need to be undertaken on behalf of London’s exotic flora.
•	There is an urgent need to increase understanding and recognition of the ecological and cultural values of exotic plants which add so much richness to our city’s environment.
•	Research into the history of London’s more significant exotic plants would be of great assistance to conservationists in communicating these values.
•	Conservationists need to recognize key problem species, some of which may be exotic, as a distinct group. Their distribution and spread need to be monitored and action taken where necessary. It is important that we do away with unhelpful native/non-native distinctions and promote management solutions to problem species on a case-by-case basis, whatever their origin.
In addition to these three major actions for the flora, the value of London’s exotic fauna should be explored.

Further Reading
Gilbert, OL (1989). The ecology of urban habitats. Chapman and Hall.
Harper, M (2000). At war with aliens. Plantlife, London.
Ingrouille, M (1995). Historical ecology of the British flora. Chapman and Hall.
Mabey, R (1973). The unofficial countryside. Collins.
Mabey, R (1996). Flora Britannica. Sinclair Stephenson.
Stace, C (1997). New flora of the British Isles. Ed.2. Cambridge University Press.

Contact
David Bevan
3 Queen’s Road
Bounds Green
London
N11 2QJ	Tel 020 8348 6005
Fax 020 8342 8754
Email work david.bevan@haringey.gov.uk
Email home conserving.bevan@virgin.net
Web http://freespace.virgin.net/conserving.bevan/	

Twenty Well-known Exotic Plants
This selected list of exotic plants found in the wild in London has been chosen to illustrate the variety of species present and the diversity of their origins.

Name	Latin Name	Origins	Description	
Buddleja 	Buddleja davidii	China 1890s	The ubiquitous butterfly bush, with its fragrant spikes of purple or white flowers, was introduced to gardens but soon escaped by means of its light wind-dispersed seeds. Though it can be encroaching it is a godsend to butterflies and moths and forms a familiar and lively sight on wasteland, old walls, banks and scrub.	
Canadian goldenrod 	Solidago canadensis	North America 1648	This vigorous relative of the scarce native goldenrod is a widespread garden escape. Its numerous golden yellow flowers can provide a colourful contrast to the purple michaelmas daisies amongst which it is often found. It provides a useful source of nectar for insects such as the common blue butterfly in the late summer but can also be invasive and may need to be controlled in some circumstances.	
Cotoneasters	Cotoneaster species	China and the Himalayas.	More than seventy species of cotoneaster are grown as ornamental trees and shrubs in gardens and parks throughout Britain. Their attractive berries are eaten by birds who thus disperse the seeds into the wild.	
Giant blackberry	Rubus armeniacus	Europe and SW Asia	The commonest bramble in the wild in London has been bird-sown from gardens. It has larger fruits than most native species, making for excellent blackberry picking in the late summer.	
Guernsey fleabane	Conyza sumatrensis	Tropical South America 1984	Originally known as Sumatran fleabane, this vigorous grey-green annual plant with heads of tiny daisy-like flowers is now frequent throughout the capital in open habitats and disturbed ground. It has greatly benefited from the “heat island effect” of the city and can grow 7 feet tall. Already starting to be recorded outside London, it seems likely to spread further as global warming and urbanisation continues	
Japanese knotweed	Fallopia japonica	Japan 1825-1840	Known sometimes as Japweed or Sally rhubarb, the shoots are eaten as a vegetable in Japan. It was much admired by the Victorians for its extraordinary vigour and architectural qualities and warmly recommended for planting in ‘the pleasure ground or by the waterside’. However, the plant is now notorious as an invader of open habitats where its dense thickets can suppress other flora. However, its abundant late summer flowers are attractive to insects and its thickets provide useful cover for birds and small mammals. Hybridisation with Russian vine has formed the Haringey knotweed, a unique plant found only at Railway Fields near Finsbury Park.	
London plane	Platanus x hispanica	Hybrid.
Parent origin: Asia Minor & N America 1680	This familiar and majestic London tree, so characteristic of the city’s squares and suburban streets, is the first intercontinental hybrid tree to have arisen. Although of obscure origin, its hybrid vigour and pollution tolerance allow it to thrive in London. Seedlings are often found though they are seldom allowed to reach maturity.	
London pride	Saxifraga x urbium	Hybrid.
Parent origin: W. Ireland & Pyrenees	A favourite London garden plant which deserves its appellation. The name, sentimentalised in Noel Coward’s song during the last war, is applied to a range of rosette forming Saxifrages bearing clouds of starry pink or white flowers on long stalks. The true London pride is a hybrid of St Patrick’s cabbage and S. umbrosa from Spain. It often escapes from gardens and can turn up in waste places, woods and by streams.	
London rocket	Sisymbrium irio	Mediterranean c.1650	Noted by 17th century botanists as growing in great abundance following the great fire of London in 1666. It apparently became extinct in the early 19th century but re-appeared in several places around the city at the end of the Second World War. This annual plant with heads of tiny yellow flowers has benefited from the warmth of central London. Although scarce and sporadic elsewhere, it can still be found in abundance on disturbed ground near the Tower of London and on the old Roman wall close by.	
Michaelmas daisies	Aster species	N America early 1700s	The purple flowers of these familiar garden plants add colour to wasteland habitats and railway embankments throughout London in the late summer. They can spread from gardens both vegetatively and through their light, wind-dispersed seeds and provide nectar for a wide range of butterflies and other insects at the end of the summer.	
Oxford ragwort	Senecio squalidus 	Southern Europe, end 1800s	Coming originally from the volcanic soils of Mount Etna in Sicily, this plant was long grown as an exotic introduction at the Oxford Botanic Garden. First recorded as an escape on the walls of that city in 1794, the plant arrived in London at the end of the 19th century after spreading along the railway system. Today, its cheerful yellow daisy flowers can be seen throughout the capital on wall tops, pavement cracks and in other such arid places. 	

Name	Latin Name	Origins	Description	
Chinese mugwort	Artemisia verlotiorum	China 1908	More widespread here than anywhere else in Britain. Tiny straw-coloured flowers open very late in year (c. November). Spreads vigorously by rhizomes. A hybrid with native mugwort has been found in London.  	
Russian comfrey	Symphytum x uplandicum	Hybrid. Parent origin UK & SW Asia	Commonest comfrey in London. Can be invasive. Provides nectar for bumblebees.	
Goat’s-rue	Galega officinalis	Europe, 1500s	‘French lilac’. Garden escape. Colourful component of wasteland, railway embankments etc..	
Ground-elder	Aegopodium podagraria	Europe, Roman times	The infamous ‘gout-weed’ or ‘bishop’s-weed’, bane of many gardeners. Introduced as a medicinal and pot herb. Useful source of food for pollen-feeding beetles and other insects.	
Himalayan honeysuckle	Leycesteria formosa	Asia	Grown in gardens for its handsome purple flowers and sweet scent. Can be found in range of habitats including ancient woodland.	
Opium poppy	Papaver somniferum	Asia via Southern Europe	Garden escape. Source of opium, morphine, codeine and heroin in warmer parts of the world.	
Broad-leaved everlasting-pea	Lathyrus latifolius	S Europe	Magenta flowers on railway embankments, roadsides and rough ground. Probably bird-sown from gardens.	
Slender speedwell	Veronica filiformis	Turkey and the Caucasus 1800s	Rock garden plant, first recorded wild in London as a weed of tennis courts in Hounslow in 1942; now frequent on suburban lawns, roadside verges, paths and churchyards. Tiny, blue flowers, seldom setting seed in Britain.	
Tree of heaven	Ailanthus altissima	Northern China, 1751	Frequently planted and thriving in streets and parks of central London. Bears attractive reddish brown keys shaped like those of the ash and sows itself about quite freely around the city.	

Annex of Latin Names 
Hybrid bluebell	Hyacinthoides x variabilis	Highclere holly	Ilex x altaclerensis	michaelmas daisy	Aster spp.	
Spanish bluebell	H. hispanica	native holly	Ilex aquifolia	purple moor-grass	Molinia caerulea	
native bluebell	H. non-scripta	Japanese knotweed	Fallopia japonica	Oxford ragwort	S. squalidus	
Bracken	Pteridium aquilinum	Londonpride	Saxifraga x urbium	Russian vine	Fallopia baldschuanica	
Buddleja	Buddleja davidii	London rocket	Sisymbrium irio	sea buckthorn	Hippophae rhamnoides	
Canadian golden-rod	Solidago canadensis	Chinese mugwort	Artemisia verlotiorum	Snowberry	Symphoricarpos albus	
native goldenrod	Solidago virgaurea	common mugwort	A. vulgaris	sticky groundsel	Senecio viscosus	
Guernsey fleabane	Conyza sumatrensis	hoary mustard	Hirschfeldia incana	Sycamore	Acer pseudoplatanus	
ground-elder	Aegopodium podagraria	parrot’s-feather	Myriophyllum aquaticum	St Patrick’s cabbage	S. spathularis	
common groundsel	S. vulgaris	floating pennywort	Hydrocotyle ranunculoides	Pyrenean saxifrage	S. umbrosa	
giant hogweed	Heracleum mantegazzianum	New Zealand pigmyweed	Crassula helmsii	American water fern 	Azolla filiculoides	
Canary Islands holly 	Ilex perado	London plane	Platanus x hispanica